1. Field of the Invention
The invention relates to a negative-working mixture which is sensitive to infrared or heat radiation and contains a polymeric binder and a substance which converts infrared radiation into heat, and a recording material comprising a substrate and a layer of this mixture. Depending on the choice of the substrate, offset printing plates or color proofs can be produced from the recording material after the imaging, which may be effected, for example, by means of an infrared laser.
2. Description of the Related Art
In the imaging with infrared lasers or thermal printing heads, negative-working layers as a rule require substantially shorter exposure times than positive-working ones. Nevertheless, only a small number of recording materials comprising a negative-working layer imageable by means of infrared or heat radiation are known to date.
EP-A 780,239 discloses a recording material for the production of offset printing plates. The negative-working layer of the recording material essentially comprises a substance which converts infrared radiation into heat, a polymeric binder which is insoluble in water but soluble in an aqueous alkaline medium and a phenol derivative. The phenol derivative contains from 4 to 8 benzene nuclei, at least one phenolic hydroxyl group and at least two groups of the formula --CH.sub.2 OR.sup.1, in which R.sup.1 is a hydrogen atom or an alkyl or an acyl group. Resins having phenolic hydroxyl groups or having olefinically unsaturated bonds are preferred as binders. Novolak resins, polyhydroxystyrenes, acrylic resins having phenolic groups and allyl methacrylate copolymers are mentioned in particular. After the infrared imaging, the recording material is developed with an aqueous alkaline solution.
EP-A 557,138 also describes a heat-sensitive, negative-working, radiation-sensitive mixtures. In one embodiment, they contain a polymer, in particular an epoxy resin, an epoxy novolak resin or an aliphatic, aromatic or heteroaromatic polyamine resin, a crosslinking agent and an acid or a compound which liberates an acid on heating. Diethylenetriamine, triethylenetetramine, diethylenepropylamine, ortho-, meta- and para-phenylenediamine, bis(4-amino-phenyl)methane and 2-methylimidazole are mentioned as crosslinking agents. In a second embodiment, the mixture contains, as essential components, a novolak, a photoinitiator which produces acid on exposure to radiation and a crosslinking agent which reacts with the novolak on heating in the presence of an acid. This photoinitiator is, for example, an ortho-nitrobenzaldehyde, an ester or an amide of 1,2-naphthoquinone-2-diazide-4-sulfonic acid or an onium salt. When this mixture is used, the recording material is first uniformly exposed so that acid is formed throughout the radiation-sensitive layer. The thermally induced crosslinking is then initiated by heat acting in a controlled manner. The unheated parts of the layer are then removed by means of an aqueous developer. For imagewise heating, an infrared laser diode, a CO.sub.2 laser or an Nd-YAG laser (1060 nm) is preferably used. The mixtures serve primarily for the production of printed circuits.
The thermally imageable recording material according to EP-A 615,162 comprises a dimensionally stable substrate and a layer which contains a cationic polymer having pendant ammonium groups of the formula --NR.sup.1 R.sup.2 R.sup.3 R.sup.4+ X.sup.- (R=H, alkyl, alkenyl or aryl), an IR-absorbing material and a dye. The material is imaged in general by means of an infrared laser or by means of laser diodes (750 to 880 nm). Those parts of the layer which are not exposed to the laser radiation are then removed by means of simple tap water or by means of an aqueous solution. The recording material is primarily intended for the production of color proofs. The substrate is then a transparent film. If images in the various primary colors (yellow, magenta, cyan and black) are arranged one on top of the other, a multicolor proof is obtained. In the production of offset printing plates, on the other hand, the substrate generally consists of metals, such as aluminum; in the production of printed circuits, it consists exclusively of paper which is impregnated with phenol resins or epoxy resins, or of polyester, polyimide or polystyrene.
U.S. Pat. No. 4,693,958 describes a recording material from which offset printing plates can be produced. It comprises a layer which can be cured by infrared radiation, for example from an Nd-YAG laser (1060 nm). The layer comprises a water-soluble or at least hydrophilic polyamide, a polymer having pendant quaternary ammonium groups or a styrene/vinylpyrrolidone copolymer. The cured parts of the layer are insoluble in water and subsequently accept the printing ink. Those parts of the layer which are not exposed to the infrared radiation are accordingly removed by means of water or an aqueous alkaline developer solution.
In addition to these directly negative-working systems, positive-negative reversal systems are also known. In these, a negative image is obtained from an actually positive-working material by an additional processing step, in particular a postbake. Examples of this are described in U.S. Pat. Nos. 5,372,907, 5,466,557 and 5,491,046, in which layers which contain a resol resin, a novolak resin, a latent Bronsted acid or an infrared absorber are imaged by means of infrared radiation (830 nm).
However, directly negative-working systems are more advantageous owing to the simpler processing. There is still a need for negative-working mixtures having as simple a composition as possible.